How comfortable are you with improvising?
My quick estimate is that the existing pattern makes a quilt 60x50", so two of them side-by-side would be 60x100". This is a bit narrower and longer than your desired size, but I would just make twice as many blocks as the pattern calls for. Lay it out, see how it looks, and remove a row or add a border as desired.
The downside of this approach is that you don't know how many setting triangles you need until the very end; if you're not comfortable with that, then you will need to do the actual math.
Good luck!